Infrared optical signature reveals the source–dependency and along–transport evolution of dust mineralogy as shown by laboratory study

Advancing knowledge of the mineralogical composition of dust is key for understanding and predicting its climate and environmental impacts. The variability of dust mineralogy from one source to another and its evolution during atmospheric transport is not measured at large scale. In this study we use laboratory measurements to demonstrate that the extinction signature of suspended dust aerosols in the 740 − 1250 cm−1 atmospheric window can be used to derive dust mineralogy in terms of the main infrared − active minerals, namely quartz, clays, feldspars and calcite. Various spectral signatures in dust extinction enable to distinguish between multiple global sources with changing composition, whereas modifications of the dust extinction spectra with time inform on size − dependent particles mineralogy changes during transport. The present study confirms that spectral and hyperspectral infrared remote sensing observations offer great potential for elucidating the size − segregated mineralogy of airborne dust at regional and global scales.

i) Reference tectosilicates and silica mineral spectra from Laskina et al. 1

Supplementary Methods a. Detailed information on the library of reference spectra
i. Reference tectosilicates and silica mineral spectra from Laskina et al. 1

-suspended aerosols
Simultaneous measurements of the aerosol infrared extinction spectra and number size distribution for three dust composing minerals (albite, oligoclase, diatomaceous earth) were obtained in the Multi-Analysis Aerosol Reactor System (MAARS 6,7 ). Aerosol particles were generated by atomizing a solution of each mineral in ultrapure water (atomiser TSI Inc., Model 3076; ultrapure water Fisher Scientific, W7-4, Lot 111718). Particles were then dried to less than 5% relative humidity. Albite and oligoclase were purchased from WARD'S Natural Science Establishment, Inc. (Items 49 V 5851, Evje, Norway and 46 V 5803, Madawaska, Ontario, Canada, respectively) and diatomaceous earth compound was purchased from Alfa Aesar (Item 89381). Minerals (albite and oligoclase) were ground using a mortar and pestle in order to get fine powder samples.
The infrared extinction spectra of the single minerals were measured in the spectral range from 800 to 4000 cm −1 at 8 cm −1 resolution by means of a Thermo Nicolet, Nexus Model 670, FTIR spectrometer. Data were reported for the 800-1400 cm −1 spectral range in the publication. The pathlength of the FTIR within the measurement volume was reported to be (78 ± 7) cm 3 . The size distribution of the suspended mineral was retrieved by combining a scanning mobility particle sizer (SMPS, TSI, Inc., Model 3936) and an aerodynamic particle sizer (APS, TSI, Inc., Model 3321). The size of particles was limited to the accumulation mode range with diameters below 2.5 µm. In their study, Laskina et al. 1 report for the different samples a mass weighted mean diameter (Dw) of (586 ± 9), (491 ± 22), and (530 ± 21) nm for diatomaceous earth, albite and oligoclase, respectively. The shape factors retrieved from SMPS and APS data were 1.34 ± 0.02 for diatomaceous earth, 1.08 ± 0.02 for albite, and 1.05 ± 0.03 for oligoclase, indicating non spherical particles.
In our analysis we consider the FTIR extinction spectra (absorbance data, log10 base) from Laskina et al. 1 as reported in their Figures 1 and 2, and the number size distribution data as reported in their Figure S1 as a function of the volume equivalent diameter. The mass concentration of each mineral is retrieved from size data in our analysis assuming spherical shape and using the densities of 2.42 g cm −3 for diatomaceous earth, 2.62 g cm −3 for albite, and 2.65 g cm −3 for oligoclase, as reported in 1 .

-suspended aerosols
The extinction spectra for selected components of mineral dust aerosol were measured in a FEP-Teflon coated steel chamber (volume of 0.151 m 3 ). The mineral dust samples investigated in Mogili et al. 2 included illite, kaolinite and mormorillonite (Source Clay Repository), quartz (Strem Chemicals (>99.5%)), hematite (Sigma -Aldrich (>99.5%)), anhydrite (Alfa Aesar (>99.0%)), and calcite. For calcite, the extinction spectra were acquired for two different samples with different size distributions, one named calcite small (OMYA products (98.0%)) and another one named calcite large (EM science (>98.0%)). The samples were held under vacuum for two hours (or were heated and pumped overnight in the case of montmorillonite for which water retention is higher) prior to the experiment to remove water. In the experimental procedure, the mineral sample was de−agglomerated and introduced into the chamber volume by using a pulsed solenoid valve coupled to an impactor plate. After a mixing time of about 10 min, the extinction spectra started to be collected continuously by means of a Mattson Infinity 60 AR FTIR spectrometer having a path length of (58.5 ± 0.5) cm in the chamber and collecting data in the range of 650-5000 cm −1 at 8 cm −1 resolution. Note that also UV VIS spectrometric measurements were performed in parallel with IR measurements during experiments to obtain mineral extinction at shorter wavelengths, and that the UV Vis spectrometer worked with a different path length. In order to account for the differences in path length between the two beams, the infrared extinction signal was scaled and normalized to a constant 100 cm path length. The FTIR measurements were performed at a time step of 53 s, corresponding to an average of 256 scans. All spectra were taken at room temperature (~296 K).
The size distribution for each mineral was not measured online but retrieved as the lognormal size distribution that allowed to best reproduce measured extinction spectra based on Mie theory calculations. The parameters of the lognormal function are reported in the In our analysis we consider the FTIR extinction spectra (absorbance data, log10 base) from Mogili et al. 2 as reported in their Figures 2 to 4. The mass concentration of the mineral samples corresponding to the measured spectra is retrieved based on the number size distribution data recalculated from the lognormal parameters as in the original publication ( Table 2 in Mogili et al. 2 ), assuming spherical shape and using the densities of 2.8 g cm −3 for illite, 2.6 g cm −3 for kaolinite, and 2.35 g cm −3 for montmorillonite, 2.6 g cm −3 for quartz, 2.97 g cm −3 for anhydrite, and 2.71 g cm −3 for calcite, in line with densities reported in 3,4 for the same minerals. For mass concentration, and effective diameter calculations reported above, a size cut off of 7.5 µm is used.
iii. Reference phyllosilicate, tectosilicates and carbonates mineral spectra from Hudson et al. 3
Experiments for each sample consisted in suspending aerosol particles of the mineral sample and then in measuring simultaneously their infrared extinction spectra by means of a Fourier-transform infrared (FTIR) spectrometer (Thermo Nicolet Nexus Model 670; liquid nitrogen cooled external MCT-A detector) and size distribution by combining an SMPS (TSI, Inc. Model 3936; DMA TSI, Inc. Model 3080, CPC TSI, Inc. Model 3025A), and an APS (TSI, Inc. Model 3321). The FTIR spectra were measured in the 800 to 4000 cm −1 spectral range at 8 cm −1 resolution by co-adding 256 scans. The aerosol samples were atomized from suspension of the clay material in ultrapure water (Optima water, Fisher Scientific, W7-4) using an atomiser (TSI, Inc. Model 3076). The illite sample was ground to get finer powder before use, whereas grounding was not applied to kaolinite and montmorillonite samples. The particles were dried to less than 15% relative humidity before entering the measuring volume by using a diffusion dryer (TSI, Inc. Model 3062).
The size distribution of the three aerosol suspensions was in the diameter range below 2 µm and the median diameter located at 153.6 nm (illite), 409.6 nm (kaolinite) and 208.8 nm (montmorillonite). For quartz and calcite, the size peaked at about 200 nm, while the peak was identified at about 50 nm for dolomite. The aerodynamic shape factors retrieved from combining mobility and aerodynamic number size measurements were 1.3, 1.1 and 1.1 for illite, kaolinite and montmorillonite, respectively, indicating non spherical particles. The experimental shape factors for quartz, calcite, and dolomite samples were determined to be 1.00, 1.05 and 0.097.
In our analysis we consider the FTIR extinction spectra (absorbance data, log10 base) as reported in Figures 3 to 5 and number size distribution data, reported as a function of the volume equivalent diameter, in Figure 1 in Hudson et al. 3,4 . The mass concentration of each mineral is retrieved from size data assuming spherical shape and using the densities of 2.8, 2.6 and 2.35 g cm −3 for illite, kaolinite and montmorillonite, respectively, and 2.6, 2.71, and 2.87 g cm −3 for quartz, calcite and dolomite, as reported in the original papers.
iv. Reference phyllosilicate mineral spectra from Dorschner et al. 5 -pellet data In their study, Dorschner et al. 5 , provided mass absorption coefficient (MAC) spectra (cm 2 g −1 ) for five phyllosilicate minerals including chamosite (St. Brigitte, France), chlorite (Switzerland), montmorillonite (Com Zorlent, Romania), serpentine (Suarum, Norway) and talc (Oberderf, Austria). The collected ground material was grounded for 6 to 12 h to obtain submicronic grains, then they were pressed into KBr pellets. Electron microscopy analysis confirmed the irregular shape and size distribution below 1−2 µm for analysed grains, with number size peaking in the submicron fraction. Transmittance spectra were measured in the range 7.7 − 14.3 µm (1300 to 700 cm −l ) by a Zeiss UR 20 spectrometer. The spectral resolution of the dataset, originally at 2 cm −1 , is degraded to 8 cm −1 in the present analysis to homogenize with the other reference datasets.
In this study we consider the MAC spectra for the five minerals reported in the original paper in their Table 3. The mass concentration of the analyzed minerals to associate to each MAC spectra in our analysis is retrieved from the following calculations. First, we consider the mineral column density in the pellet as reported by the authors to be 3.18 10 −4 g cm −2 for all minerals investigated, with the exception of talc for which it was 3.18 10 −5 g cm −2 . The mass concentration in the pellet is retrieved assuming that the original disk is 1 mm thick and it has a surface of 1 cm 2 , and by dividing the total column density by the thickness of the pellet, which results in a mass concentration of 3.18 10 6 mg m −3 (3.18 10 5 mg m −3 for talc) in the pellet. Based on mass concentration data we estimate that the ratio between MAC spectra in cm 2 g −1 and absorbance spectra for Dorschner et al. 5 data is equal to a multiplicative factor of 7421. Given that MAC spectra are used in the deconvolution instead of absorbance spectra, as done for the other reference minerals, we account for this multiplicative factor in the mass concentration values for Dorschner et al. minerals when converting deconvolution coefficients Ci(t) from LSM analysis into mineral mass concentrations. This results in a mass concentration of 2.30 10 10 mg m −3 (2.30 10 9 mg m −3 for talc) to consider for this analysis.
v. Reference tectosilicate mineral spectra from Caltech -pellet data The extinction spectrum for orthoclase feldspar is obtained from the Caltech Mineral Spectroscopy server (http://minerals.gps.caltech.edu/files/Infrared_MIR/Minerals_From_JK/Index.htm). Data correspond to KBr pellet spectroscopic measurements in the range 450 to 4400 cm −l and were obtained by means of a Perkin Elmer FTIR Spectrometer. No information on the origin of the sample, size distribution and mass concentration of the material in the pellet is provided. In order to reconstruct a reasonable mass concentration to associate to the spectra we take as reference value a mineral column density of 3 10 −4 g cm −2 in the pellet (a quite typical value in pellet spectroscopy corresponding to about 300 µg material diluted in 300 mg of KBr, also in line with the density reported in Dorschner et al. 5 ), which corresponds to 3 10 6 mg m −3 mass concentration assuming a usual pellet of 1 mm thick and with a surface of 1 cm 2 .
In our analysis we consider the FTIR transmission spectra (provided as % values from the Caltech Mineral Spectroscopy server) which we convert in absorbance assuming a log10 base.
vi. Additional reference spectra for gaseous compounds and ammonium sulfate aerosols Extinction spectrum for gaseous compounds possibly present in the CESAM chamber during dust experiments and therefore visible in the FTIR signature have been added to the library. These include CO2 and H2O. For those species the spectra are retrieved from the HITRAN (high-resolution transmission molecular absorption database, https://hitran.org/about/), taking data at 300°C and 0.5 cm −1 resolution. The extinction spectrum of ammonium sulfate particles was acquired for suspended aerosols in CESAM and reported in Di Biagio et al. 8 has also been included as control species, as discussed in the main manuscript.

b. Mass concentration from LSM: uncertainty calculations
An error analysis is performed to constrain the uncertainty on the estimated single mineral and total mass concentration of dust obtained from the LSM procedure. The uncertainty in dust mass concentration from LSM arises from a combination of five uncertainties: 1/ uncertainty on CESAM absorbance spectra (±1% noise) 8 and 2/ CESAM optical path (±0.3%) 8 ; 3/ uncertainty on reference mineral absorbance spectra (±1% noise) and 4/ optical path (±5%) (value at the average of those reported by 1-4 ); 5/ uncertainty on the mass concentration of single reference minerals (±10%; estimated considering size distribution uncertainties and pellet column to volume concentration calculations detailed in the previous section). We do not take into account uncertainties on the coefficients Ci arising from the LSM procedure itself. Under the hypothesis that the sources of uncertainty are independent, the 90% Confidence Interval (CI) from Gaussian standard distribution is calculated by adding in quadrature all mentioned uncertainties: 90%CI=±1.645 (1%) +(0.3%) +(1%) +(5%) +(10%) =±19% (1)

c. Details on calculations for comparison of LSM and XRF mass concentration data
The XRF mass concentration, i.e. the ratio of the mass deposited on the filter (µg) to the total volume of sampled air (m 3 ), is integrated over the dust sampling time on filter. In order to compare to XRF, the LSM mass concentration is averaged over the entire filter sampling period.
Since LSM data refer to dust inside the CESAM volume, the mass concentration from XRF is corrected for particle losses along sampling lines from the chamber to the filter following the results in Di Biagio et al. 8 . Being L(D) the loss fraction (0-1) as a function of the particle diameter from the CESAM chamber to the sampling filter, the mass-weighted total loss integrated over the XRF filter sampling period (LXRF) is calculated as: where dN(Di,t) is the number size distribution inside the CESAM chamber as retrieved from SMPS and OPC measurements and ρ is the dust density (set at 2.5 g cm -3 following literature 9,10 ). The XRF mass concentration is then divided by (1 -LXRF) to correct for particle losses along filter sampling lines. The LXRF term varies between a minimum of 29% for Kuwait and Morocco to a maximum of 67% for the Arizona sample, in relation with the abundance of the coarse mode in the dust aerosols. As reported in Table 1 in the main manuscript, Kuwait and Morocco show amongst the lowest Deff within the analysed samples and Arizona Deff is amongst the highest values in the dataset. The uncertainty on the loss-corrected XRF mass concentration is estimated with the statistical error propagation formula taking into account the uncertainty on XRF mass concentration (±10% from comparison to gravimetric analysis in Caponi et al. 11 ) and the uncertainty on LXRF (between ±20 and 26%, accounting for a sizeaveraged 20% uncertainty for L(Di) 8 and standard deviation of the average over sampling time in Eq. 2). The total uncertainty on the loss-corrected XRF mass concentration is in between 23 and 27%.

Supplementary Tables
Supplementary Table 1. List and origin of the dust natural samples analyzed in this study, as reported in Di Biagio et al. 8 . The collection coordinates indicate the location where the soil sample used for aerosol generation was originally collected. Supplementary Table 3. Information on reference extinction spectra used in this study. The table reports the mineral, its mineralogical family association, size distribution information on analysed samples, the mass concentration associated to the reference extinction spectrum, the path length of spectroscopic measurements, the resolution of acquired spectra, and the reference paper for the dataset. Data are reported as absorbance (using log10) for all reference studies with the exception of Dorschner et al. 5 for which data are MAC (mass absorption cross−section, cm 2 g −1 ). All measurements were acquired at 8 cm −1 resolution, with the exception of Dorschner et al. 5

Supplementary Figures
Supplementary Figure 1. Illustration of Linear Spectral Mixing (LSM) analysis. Measured and modelled extinction spectra (expressed as absorbance in log10 convention) at the peak of the dust injection in the CESAM chamber and after 20 and 60 min aging time. The modelled spectra from LSM analysis is the sum of the single minerals contributions (Sum Minerals, plotted in red) and baseline offset term (Baseline, plotted in black dotted). The Residual (difference between measured and modelled signal) is also shown (plotted in grey). Examples of baseline and residuals are reported at the bottom of each plot in reference only to the spectra at 60 min. The horizontal black line represents the 0 value. Data are shown for all samples with exception of Tunisia, Australia and Kuwait for which the plots are already reported in Figure 1 in the main manuscript.
Supplementary Figure 2. Library of reference extinction spectra. Single mineral reference spectra (normalized by mass) for the different families considered, including phyllosilicates, tectosilicates, carbonates, sulfates, and amorphous silica. For all different reference studies, with the exception of Dorschner et al. 5 , data are absorbance divided by total mass concentration, µg m −3 . For Dorschner et al. 5 data are MAC (mass absorption cross−section), cm 2 g −1 , divided by total mass concentration, µg m −3 . Data from Dorschner et al. 5 (chlorite, chamosite, montmorillonite, serpentine), Caltech (orthoclase), and Laskina et al. 1 (diatomaceous Earth) are plotted versus right side y−axis.

Supplementary Figure 3. Scatterplot of LSM versus XRD mineral mass percent abundances.
Retrieved LSM percent content in mass of phyllosilicates, calcite, feldspars and quartz plotted against estimations from XRD. The solid line is the 1:1 line, the dotted and dashed lines represents ±10% and ±25% in mass percentage. Error bars represents LSM and XRD uncertainties. The tendencies in LSM-XRD differences in our dataset are in line with those found for sedimentary rocks investigations 12 .